NativeCurrency must Time-Lock Part of Coinbase

[aszepieniec]

The type script NativeCurrency must time-lock for three years (or whatever lock time) half (or whatever fraction) of the amount listed in field coinbase. Recall that Block::is_valid verifies that the coinbase is not larger than the block subsidy. This change would therefore establish that whenever a miner produces a block and generates new coins, half (..) of those coins are time-locked.

Since there is going to be a separate guesser fee, the mechanism that turns guesser fees into UTXOs must also timelock half (..) of the guesser fee. Otherwise it would by possible to bypass the lock time by collecting the block subsidy through guessing the block instead.

This issue supersedes #217 .